Fix for bug #311175 and bug #311179: Clean up python support
app-editors/vim and app-editors/gvim both rely completely on vim.eclass, so this
is the place to ensure we only use python2, since vim does not yet support
python3.

Big long-overdue cleanup:
- Removed all code that had anything to do with <vim-7 since all version 6 has
been out of the tree for over a year now.
- Reorganized IUSE and all [RP]DEPEND variables to be less spread-out
- Moved in all current IUSE and [RP]DEPEND from app-editors/[g]vim[-core]
- Make dev-util/ctags dependent on USE=minimal for app-editors/vim
(Bug #261025)
- Don't call update_vim_symlinks() for app-editors/vim-core
(Bug #252724)

Allow either gzipped or uncompressed patches in the tarball.
Also removed date-seeking regexp to find first and second lines of the patch
since as of 7.2.167 the date format in all vim.org patches has changed. It is
less work (while marginally less correct) to just look for lines that start with
'***' and do not end with '****' (and "---" / "----" for the second line).

remove code for handling versions which are no longer in the tree. add in a check that we're trying to build a version of vim which this eclass can actually handle, in case anyone has a silly overlay/package.mask setup.

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